To provide for the recognition of the two addressing tones a unique logic circuit is provided to recognize the particular tones arriving in a predetermined sequence. This differs from the tone unlatching systems of the prior art in which the arrival and detection of the first tone unlocks the second tone detector. It will be appreciated that in the prior art systems, decoding is accomplished when there is an output signal from the second tone detector. In the present invention, input signals are simultaneously applied to two filters tuned to the different tones. The outputs of these filters are coupled to a logic circuit which responds only to the desired address sequence. This logic circuit also permits group calls in which a number of pagers are addressed simultaneously by the elongation of the first tone. The decoding section of the subject tone decode circuit takes on the form of timing circuits which are normally-inoperative and are rendered operative only during the presence of a particular tone. This saves considerable battery power and is unlike any prior art tone detection system which operates just exactly the reverse by having normally-on decoders. Moreover, shock protection is provided by an override circuit which inhibits the output from one timing circuit when the other tone is present. This prevents false addressing when the pager is, for instance, banged against the side of a desk. The reason for the shock protection is that the filters are tuning fork type filters which, if subjected to mechanical shock, as by the bumping of a pager, produce output signals. These signals may falsely indicate the predetermined address.
While the subject invention resides in address tone decoding, a complete series of paging systems will be described which includes a hard copy pager with a "print-on-the-fly" feature, a soft copy pager which utilizes a precessing display to display only a portion of the received message while the message moves across the display, and two message entry units. Also described is a digital address decoding system to be contrasted with the subject tone address decoding system.